1. Field of the Invention
The present invention relates to an absolute piezo-resistive pressure sensor or transducer. More particularly, the invention relates to an apparatus and method for minimizing drift of an absolute piezo-resistive pressure sensor due to the progressive release of mechanical stress over time.
2. Description of Related Art
Absolute micro pressure sensors have a relatively thin silicon deflectable membrane that mechanically deflects under pressure. The mechanical deflection of the membrane is correlated to a pressure being measured on one side of the membrane. In order to measure an absolute pressure, the opposite side of the membrane is exposed to a substantially constant pressure created at the time of manufacture by a hermetically sealed cavity.
FIG. 1 is a cross-sectional view of a prior art absolute piezo-resistive pressure sensor. Such a piezo-resistive sensor measures the mechanical deflection of a membrane or diaphragm by measuring a change in electrical resistance of a piezo-resistive film deposited or diffused on the membrane. A silicon wafer or die 10 is etched to form a cavity 25 closed at one end by a relatively thin deflectable membrane or diaphragm 40. One side of the membrane 40 is exposed to a pressure being measured (Pmeasured) within the cavity 25, while an opposite side of the membrane 40 is subject to an absolute or substantially constant reference pressure (Pref) within a hermetically sealed chamber 35 filled with gas. A piezo-resistive material or film 30 is deposited or diffused on the surface of the membrane within the hermetically sealed chamber 35. The silicon wafer 10 is assembled to a glass base plate 5 so that the membrane 40 is substantially aligned with an opening or hole 20 defined in the base plate 5.
Anodic bonding is a well known method for hermetically and permanently bonding silicon to glass without the use of adhesives. Silicon and glass components are heated to a temperature typically in the range of between approximately 300° C.-approximately 500° C., depending on the glass type. At such temperatures, alkali metal ions in the glass become mobile. A relatively high voltage (e.g., approximately 250 V-approximately 1000 V) is applied across the components as they are brought proximate or in contact with one another causing the alkali cations to migrate from the interface resulting in a depletion layer with relatively high electric field strength. The resulting electrostatic attraction brings the silicon and glass components into intimate contact with one another. Further current flow of the oxygen anions from the glass to the silicon results in an anodic reaction and hence a permanent chemical bond at the interface between the two components without the need for a bonding interface material or adhesive.
FIG. 3 is a side view of the sensing element of FIG. 1 assembled to the glass base plate using an alternative eutectic solder alloy (e.g., 60SN40Pb solder alloy). This process requires the bonding surfaces to be plated with noble metals in layers preferably of only fractions of a micron thick. The mechanical stress resulting at the assembly interface and therefore at the deflectable membrane occurs over time due to aging effects regardless of the type of assembly method or process used. Another factor contributing to the overall mechanical stress results from temperature variations if the coefficient of thermal expansion of the sensing element and base plate (substrate) are not matched.
In order to protect its electronic circuitry against malfunction when monitoring pressure within certain environments the piezo-resistive pressure transducer or sensor shown in FIG. 1 is encapsulatd in a hermetically sealed packaging or enclosure 50 depicted in FIG. 2. EP Patent No. 1 184 351 discloses a method for brazing two glass components together in order to form leak tight container for encapsulating electronic components such as a pressure transducer implanted in a human body. Enclosure 50 formed by cover 55 and base plate 5 are brazed together forming a hermetic seal. The enclosure is preferably made of glass and filled with a gas or fluid at a substantially constant pressure P1. Most preferably, glass enclosure 50 is filled with an inert gas such as helium or argon in order to prevent or minimize oxidation/aging of electronic circuitry 65 disposed therein. Another factor in the selection of the gas or fluid for filling enclosure 50, is that it preferably be compliant with a leak tester used for testing the implant hermeticity after encapsulation. The capsule or packaging is placed in a hermetically sealed helium chamber subject to a substantially constant pressure preferably in the range of approximately 100 mbar-approximately 1000 mbar to detect any helium molecules escaping or leaking from the capsule.
Over time the pressure sensor measurement response of a conventional absolute piezo-resistive pressure sensor will undesirably drift due to unwanted mechanical stress. It is therefore desirable to develop an improved absolute piezo-resistive pressure sensor that calibrates and then compensates for pressure sensor drift over time due to mechanical stress.